Half of Andromeda’s satellite galaxies orbit in a mysterious disk

Disk defies easy explanation, but may be aligned with Milky Way.

Thirteen of Andromeda Galaxy's satellites lie in a remarkably flat plane and orbit in the same direction, as depicted in this illustration.

Ibata et al.

Large galaxies such as the Milky Way have many smaller satellite galaxies, bound to them by gravitation. According to the widely accepted theories of galaxy formation, these satellites were leftovers from the slow merger process that made the larger galaxies. We'd expected these satellite galaxies to be evenly distributed around the Milky way, but recent studies showed that many of them lie close to a single plane, tilted with respect to the Milky Way's disk. New observations may have revealed a similar structure of satellites around our closest large neighbor M31, also known as the Andromeda Galaxy.

Rodrigo A. Ibata and colleagues found that nearly half of M31's known satellites lie in a single, relatively thin plane. They also measured the motion of these galaxies and found they were revolving in the same direction around Andromeda. These results—as with the previous Milky Way observations—were contrary to expectations, in which satellites would be distributed more or less spherically and moving in random directions.

A complete census of Milky Way satellites is difficult to achieve. Since we are located within the Milky Way's disk, a large part of the sky is obscured by our own galaxy, which may block the view of several small galaxies. While a few examples of the satellite galaxies are large—the Magellanic Clouds for the Milky Way and the Triangulum Galaxy (M33) for Andromeda—most are small and faint, increasing the challenge of locating them all.

The Pan-Andromeda Archaeological Survey (featuring the diverting acronym PAndAS) was established to provide a high-resolution, large-scale panorama of M31 and its environs. 27 dwarf galaxies that can be unambiguously associated with Andromedalie within the PAndAS survey region. The astronomers measured the distances and velocities of each of these galaxies, yielding a three-dimensional and dynamical view of the M31 system.

They found 15 of those satellites were arranged along a relatively thin arc from the perspective of Earth, meaning they lie close to a single plane. Further analysis revealed 13 of the 15 galaxies were also moving in a coherent pattern: those "north" of Andromeda were moving away from us, while those "south" were traveling toward us. That indicates a clear rotational pattern; the authors estimated only a 1.4 percent probability of motion like this being random chance.

As with most things in galactic astronomy, the dynamics are somewhat messy. The disk is relatively thin compared to a sphere surrounding M31, but it's still 46,000 light-years thick—nearly 15 times thicker than Andromeda's galactic disk. Nevertheless, the structure appears real: it's extremely improbable that galaxies would arrange themselves in such a fashion under the ordinary action of gravity.

According to the theory of structure formation, galaxies collapsed gravitationally in roughly spherical shapes, with rotation in large spiral galaxies producing the flat disk structures. Satellite galaxy motion would be random, since they form independently of each other before coming close enough to be captured via gravity. The standard versions of galaxy formation—along with modified gravity schemes—are currently unable to explain flat, rotating structures composed of dwarf galaxies. It is possible that these satellites must have formed together in some way, since they are all relatively ancient (based on the populations of stars they contain).

But there is an alternate possibility, given that the disk of galaxies is nearly aligned with the Milky Way. While the authors are (wisely) reticent to guess why that might be, the alignment suggests the possibility that the two biggest galaxies in the Local Group (the structure containing M31, the Milky Way, and a number of other much smaller galaxies) affected the formation or orientation of each others' satellites. Assuming the alignment isn't coincidental, it's possible that this idea can be formulated as a coherent physical model. Not all of M31's satellites are part of the structure, though, so things remain (as the kids say) complicated.

Another caveat: we only have extensive satellite galaxy data on M31 and the Milky Way, but there are 100 billion large galaxies in the observable Universe. Without further data, there is no way to determine whether disk-like structures are common or not. Nevertheless, their presence in both Andromeda and the Milky Way hint that they may be present elsewhere. Further work is necessary, both on the theoretical and observational sides, to resolve the questions these findings raise.

Promoted Comments

Question for the astronomers in the group: Why is it that we expect planets orbiting a star to do so mostly within a single plane, but we do not expect the same of small galaxies orbiting large galaxies? Do systems of galaxies not also form in a similar "disks of dust around a strong gravity point" method as solar systems?

These disks occur because the planets are formed at the same time as the sun, kind of like an excretion of excess materials. These small galaxies are thought to have been captured by gravity as they passed by.

Yeah, add to that while reading the news this morning a study came out suggesting that there are 100 billion planets in our galaxy alone, and the numbers of possible worlds out there in that vast sky get truly astronomical (bad pun intended).

It's so sad to think that even Andromeda is so far away that we'd never even be able to get any kind of message from it, let alone travel. Space kinda sucks, though it is cool to see Andromeda through a telescope (as unimpressive as it really is with human eyes)

Is anyone else completely blown away by the sheer magnitude of this, or is it just me?

Watch the series on the Discovery Channel about how the universe was made or how it works, I forget the name. After seeing that you can't help but be blown away.

The strongest feeling I get when I think about this stuff is how petty life is.

Watch Cosmos (it's on Netflix streaming, if that helps). The one I saw from the Discovery channel was narrated by Mike Rowe, and it moved incredibly slow.

There's also the book. I haven't read Cosmos yet, so I'm not sure how well it syncs up with the show. But I've read other stuff Sagan wrote, and he's very good. Accessible, on track, accurate, and enjoyable to read.

As an atheist myself, the universe is something as close to God for me as I ever marvel.

What you describe is the logos of the ancient Stoics.

Quote:

In Stoic philosophy, which began with Zeno of Citium c. 300 BC, the logos was the active reason pervading and animating the universe. It was conceived of as material, and is usually identified with God or Nature.

Is anyone else completely blown away by the sheer magnitude of this, or is it just me?

Watch the series on the Discovery Channel about how the universe was made or how it works, I forget the name. After seeing that you can't help but be blown away.

The strongest feeling I get when I think about this stuff is how petty life is.

Watch Cosmos (it's on Netflix streaming, if that helps). The one I saw from the Discovery channel was narrated by Mike Rowe, and it moved incredibly slow.

There's also the book. I haven't read Cosmos yet, so I'm not sure how well it syncs up with the show. But I've read other stuff Sagan wrote, and he's very good. Accessible, on track, accurate, and enjoyable to read.

As an atheist myself, the universe is something as close to God for me as I ever marvel.

Then you might find the theologian Paul Tillich's definition of God somewhat interesting (from the introduction of his three-volume Systematic Theology). God is the ground of all being. The ultimate concern (with the denotative meaning of the word ultimate) whatever that might be.

So for you, the Universe itself is the ground of all being, the ultimate concern. For theists, it would be just one more feature - the ground of all being is sentient. But we all can agree that whatever IS the ground of all being is vast beyond comprehension and always will be (at least until someone comes up with the unicorn known as the Theory of Everything). Do atheists and theists really have to behave like Congress? Awe is not the exclusive domain of believers.

Well, let's see. M31 is about 500,000 light years in diameter, and is 2,500,000 light years distant. So, the diameter is 20% of the distance - a fairly large proportion. I would think our mutual gravitational influence would be fairly large, so aligning the orbits of satellite galaxies and clusters isn't too hard to imagine.

Question for the astronomers in the group: Why is it that we expect planets orbiting a star to do so mostly within a single plane, but we do not expect the same of small galaxies orbiting large galaxies? Do systems of galaxies not also form in a similar "disks of dust around a strong gravity point" method as solar systems?

I am just a dumb civilian on this topic, but disc-shaped rotations of small galaxies around a large one is intuitive to me. Don't rotation and gravity usually end up with discs? Our solar system is a disc. Our galaxy is a disc. Why wouldn't small galaxies rotating around a larger one also end up in a disc pattern?

Maybe the Big Bang produced a spherical region of matter, but don't gravity and rotation push astronomical bodies toward discs?

IMHO disc formation is a function of the rotation of the gas cloud that produces the solar system or galaxy. What bothers these guys is the fact that the satellite clusters are orbiting at right angles to the plane of the galaxy itself. Actually we sorta have the same problem with the Magellanic clouds. That's not particularly intuitive, to me.

Question for the astronomers in the group: Why is it that we expect planets orbiting a star to do so mostly within a single plane, but we do not expect the same of small galaxies orbiting large galaxies? Do systems of galaxies not also form in a similar "disks of dust around a strong gravity point" method as solar systems?

These disks occur because the planets are formed at the same time as the sun, kind of like an excretion of excess materials. These small galaxies are thought to have been captured by gravity as they passed by.

Yeah, add to that while reading the news this morning a study came out suggesting that there are 100 billion planets in our galaxy alone, and the numbers of possible worlds out there in that vast sky get truly astronomical (bad pun intended).

the authors estimated only a 1.4 percent probability of motion like this being random chance.

In other words, it's not really all that improbable that the apparent structure is actually the result of random chance.

Somewhat less than 5 sigma, yes.

Though the idea that the Milky Way causes some random paths to Andromeda to be more likely than others makes some intuitive sense.

Yet in the same article they mention that the group of galaxies should be moving randomly because their formation away from one another invites little gravitational interaction? Or am I misreading that? But if that is the case, how much less influence would our galaxy have being farther than the primary (Andromeda) of that group?

Question for the astronomers in the group: Why is it that we expect planets orbiting a star to do so mostly within a single plane, but we do not expect the same of small galaxies orbiting large galaxies? Do systems of galaxies not also form in a similar "disks of dust around a strong gravity point" method as solar systems?

You need friction between orbiting elements in order to collapse them into a disk under such a disk-making process.

Planetary systems start out as clouds of gas/dust. Each particle orbits the common center of gravity. But the orbits are all randomly aligned. On average, though, there is some net angular momentum (this is just a statistical likelihood). If you were to observe such a system over time, early on you would just see a roughly spherical dust cloud. Very fine measurements might allow you to discern what that net angular momentum is, but is would be hard to tell from just looking.

As the particles interact over time, you would notice that the whole cloud starts to look like its rotating in the same direction. This is because repeated interaction between particles tends to average out their angular momentum. Example: two particles orbiting opposite directions crash into each other and stick together. One was orbiting faster in its direction, so the net result is momentum in the direction of orbit of the faster particle. Repeat this billions of times, and eventually everything is going the same direction (at the same time, many particles will lose so much angular momentum in their interactions that they no longer orbit the center. Instead, they fall into the center where they become part of the star.)

This effect does not entirely rely on particles sticking together. If you think of those particles as a gas, then friction within the gas will tend to make it all flow in the same direction over time.

Now, here's the trick. The reason such a cloud stays as a (rough) sphere at the beginning is because all of the particles are orbiting in random directions. As particles start to orbit in the same direction, the sphere begins to collapse into a disk. As more interactions occur, the particles further average-out their orbital directions, and fewer particles are in orbits that take them out of the disk. Eventually you get a pretty thin disk. The thinness of the disk will be limited by thermal interaction among the particles (in essence, jostling keeps them from getting any closer together). In the case of a solar system, the planets form after this disk forms, which is why you get the planets all aligned in an eliptic.

Further out from the central star, you would not get enough interaction to collapse into a disk. Instead, you'd get local collapsing into small bodies, each one of which would have a randomly aligned orbit around the common center of gravity. This is why the Oort Cloud is thought to be a sphere, rather than a disk. There was not enough particle density to have enough friction to collapse into a disk.

It works the same for spiral and lenticular galaxies (the ones where most of the mass is also on an eliptic). Galaxies don't get as thin along their eliptics as solar systems do. The primary reason is that as the gas cloud collapsed into a disk, stars started to form. Once you have the masses concentrated in that manner, the contribution of friction declines (stars just don't crash into each other very often, and their momentum isn't affected very much by interaction with galactic dust) and the process of collapse almost completely stops.

In the galaxy case, satellite galaxies would form similarly to Oort Cloud objects. There would not have been enough interactions to force those satellite galaxies into having the same angular momentum around the common center of gravity.

To finish out the galaxy discussion, eliptical galaxies stay eliptical because there is not enough friction in the system to collapse into a disk. Again, stars aren't affected enough by galactic dust and don't crash into each other often enough for them to average out there angular momentum. Eliptical galaxies are believed to form in one of two ways.

(1) During original formation, a galaxy developed enough density for lots of star formation early enough that the gas cloud hadn't collapsed into a disk first. Once enough gas has collapsed into stars, the friction process becomes negligible and the disk-making process stops. This may be why these galaxies end up as ellipses and not more spherical. They partially collapsed before the friction process got to negligible levels.

(2) Two galaxies of roughly equal size merged, and gravitational interactions sent the stars in all kinds of random orbits. Once in random orbits, again, there's not enough friction for collapse back into a disk. In this case, the reason we end up with an ellipse is not because of partial collapse, but merely because the orbits are not entirely random. They will tend to be dominated by the orbital momemtums of the two galaxies. Presumably, if two galaxies of equal size merged whose eliptics were perfectly 90 degrees offset, you might get a spherical galaxy. (Note that a big galaxy can swallow a small galaxy without affecting the shape of the bigger galaxy, however. Gravitational interactions and what little friction there is will tend to bring the smaller galaxies' stars into the bigger galaxy's disk.)

the authors estimated only a 1.4 percent probability of motion like this being random chance.

In other words, it's not really all that improbable that the apparent structure is actually the result of random chance.

Somewhat less than 5 sigma, yes.

Though the idea that the Milky Way causes some random paths to Andromeda to be more likely than others makes some intuitive sense.

Yet in the same article they mention that the group of galaxies should be moving randomly because their formation away from one another invites little gravitational interaction? Or am I misreading that? But if that is the case, how much less influence would our galaxy have being farther than the primary (Andromeda) of that group?

The article doesn't say that. It just says they form independently, so their motion should be random. See my post above for explanation of how we believe galactic and solar system disks form

Ha, thank you, when I first heard this I seemed to remember a similar ring around MW. [But I can't find the result now. :-/]

The intuitive idea is that it has something to do with DM, as the null hypothesis simulations likely use isolated galaxy models. Maybe this is caused by a heretofore unobserved DM filament threading the Local Group and especially its two dominant galaxies. I assume the group should have at least one such filament associated with it.

The mutual influence idea is somewhat less generic, and would depend on the rings being newer galaxies. It predicts the seeming older more erratic population, but a filament that the LG orients with over time does that too.

Perphenazine wrote:

Question for the astronomers in the group: Why is it that we expect planets orbiting a star to do so mostly within a single plane, but we do not expect the same of small galaxies orbiting large galaxies? Do systems of galaxies not also form in a similar "disks of dust around a strong gravity point" method as solar systems?

whquaint wrote:

I am just a dumb civilian on this topic, but disc-shaped rotations of small galaxies around a large one is intuitive to me. Don't rotation and gravity usually end up with discs? Our solar system is a disc. Our galaxy is a disc. Why wouldn't small galaxies rotating around a larger one also end up in a disc pattern?

I'm not an astronomer, but let me try this on:

The generic form expected from a system with a potential, here gravitational, is a ball. We can see that in the old spherical clusters that are the most ancient structures of galaxies. As well as in the many elliptic galaxies that are presumed to result from large mergers.

This is the case when you have no net angular momentum and the system is still kinetically "hot". It approaches an equipartition theorem situation of thermodynamic statistical physics, systems with a statistically well defined heat. (But in physics term not quite, it is actually the virial theorem that applies to systems constrained by a potential.)

When gravitational systems are cooled down by dissipative forces, a random net angular momentum will be visible and the ball collapses gravitationally (still under dissipative cooling) to a disk. This is what happens in disk systems like spiral galaxies and protoplanetary disks.

the authors estimated only a 1.4 percent probability of motion like this being random chance.

In other words, it's not really all that improbable that the apparent structure is actually the result of random chance.

That is only the probability for the motion to be oriented in that way, not the probability for the disk-like spatial distribution. Combine that and the probability of randomly forming this structure would be far lower.

Which, I read somewhere, is perhaps on the way to be updated to ~ 200 billion large galaxies. Mind blownier.

ws3 wrote:

Quote:

the authors estimated only a 1.4 percent probability of motion like this being random chance.

In other words, it's not really all that improbable that the apparent structure is actually the result of random chance.

With two rings we get ~ 10^-4 likelihood for independent random chance, which translates to ~ 4 sigma.

(The alignment with MW bumps it up, something similar for the MW would do so irrespective of a hypothesis constraint on the alignment.)

If you come up with a hypothesis, you can test it at 3 sigma.

whquaint wrote:

Maybe the Big Bang produced a spherical region of matter,

Just in case you didn't mean it literary: inflationary standard cosmology can't produce such regions other than by chance.

The cosmological process produces an almost completely uniform distribution of matter. It is seeded with fluctuations that appears in, and is blown up, by the inflation field. It is these fluctuations that drives later structure formation, wherein collapsing gas clouds and then galaxies appears.

The generic form expected from a system with a potential, here gravitational, is a ball. We can see that in the old spherical clusters that are the most ancient structures of galaxies. As well as in the many elliptic galaxies that are presumed to result from large mergers.

This is the case when you have no net angular momentum and the system is still kinetically "hot". It approaches an equipartition theorem situation of thermodynamic statistical physics, systems with a statistically well defined heat. (But in physics term not quite, it is actually the virial theorem that applies to systems constrained by a potential.)

When gravitational systems are cooled down by dissipative forces, a random net angular momentum will be visible and the ball collapses gravitationally (still under dissipative cooling) to a disk. This is what happens in disk systems like spiral galaxies and protoplanetary disks.

No.

Your understanding of thermodynamics is not wrong, but your application to large-scale astronomical processes is wrong. Ball-shaped structures like galactic clusters do not stay as balls because of thermodynamic equilibrium. And they do not stay that way because they have zero net angular momentum. They stay that way because there is not enough interaction between elements for angular momentum to be transfered between objects, such that over time each object's momentum would approach the average.

And collapse into a disk does result in cooling, but is not because of it.

Ball-shaped structures like galactic clusters do not stay as balls because of thermodynamic equilibrium. And they do not stay that way because they have zero net angular momentum. They stay that way because there is not enough interaction between elements for angular momentum to be transfered between objects, such that over time each object's momentum would approach the average.

I think we are saying the same thing. I specifically mentioned the appropriate virial theorem as contrast, but if I was unclear, I apologize.

Chuckstar wrote:

And collapse into a disk does result in cooling, but is not because of it.

Again I believe we are saying the same thing re friction/dissipative "cooling" of kinetic energy.

Your detailed knowledge how dissipation can arise is appreciated. Protoplanetary disks add mechanisms, and those are who I know better (astrobiological interest).

Yeah, add to that while reading the news this morning a study came out suggesting that there are 100 billion planets in our galaxy alone, and the numbers of possible worlds out there in that vast sky get truly astronomical (bad pun intended).

As an atheist myself, the universe is something as close to God for me as I ever marvel.

That is pantheism, not atheism.

You can cut out the mysticism/middle man, and just appreciate nature for what it is - awesome and with the necessary cats.

ewelch wrote:

But we all can agree that whatever IS the ground of all being is vast beyond comprehension and always will be (at least until someone comes up with the unicorn known as the Theory of Everything).

No, we can't agree on that. And we shouldn't, for good reason.

The ground is physics, and science shows it is comprehensible. For example, the Higgs field, modulo if it is a standard Higgs particle field, just finished our complete understanding of the laws of everyday physics.

To mix up the basis for the universe with the object itself is mysticism and (as "the ground of all being", "vast beyond comprehension" and "always will be") a theological deepity claim (as Dennett would label it).

As for the rest, we already have anthropic theory and its successful predictions such as the value of cc. TOE has to prove itself in competition.

ewelch wrote:

Do atheists and theists really have to behave like Congress? Awe is not the exclusive domain of believers.

Again I believe we are saying the same thing re friction/dissipative "cooling" of kinetic energy.

I believe we are implying different cause and effect. Systems do not collapse into disks because of cooling. The same phenomena that drive collapse also drive cooling. Friction and inelastic collisions both result in each particle's angular momentum approaching the average*. And friction and inelastic collisions both also result in conversion of potential and kinetic energy to heat, which is then radiated away from the system. But the collapse and the radiation are both effects. Neither cause the other.

*"Each particle's angular momentum approaching the average" is not actually a correct description of the end-point of the disk-making process, but I think people generally understand the shorthand when I say it that way. Basically, what you really end up with is momentum transferred between objects (atoms, molecults, particles of dust, etc.) until each object ends up in a relatively circular orbit in the same eliptic as the other objects (such that further jostling and momentum transferring is limited). Because objects have different masses and their velocities depend on distance from the center, it is not accurate to say that their momentums will be the same.

the authors estimated only a 1.4 percent probability of motion like this being random chance.

ws3 read this as implying the rotating disk-like distribution is not improbable (more than a 1 in 100 chance). However, this 1.4 percent probability is not the probability or a random distribution appearing to be a rotating disk-like structure, it is simply the probability of dwarf galaxies in a disk-like structure (which itself may be improbable) to have random velocities consistent with the observed net rotation. I was simply trying to make the point that, once the spatial distribution is factored in, the observed rotating disk-like structure is inconsistent with a random distribution at far greater than the 1.4 percent level.

Yeah, add to that while reading the news this morning a study came out suggesting that there are 100 billion planets in our galaxy alone, and the numbers of possible worlds out there in that vast sky get truly astronomical (bad pun intended).

Yeah and just think if only 10% of those 100 billion planets in our galaxy alone contained as much sentient life as our planet does (~7 billion maybe more if you count whales, apes, etc.). The number of "people" in the galaxy would be about 100 billion!

the authors estimated only a 1.4 percent probability of motion like this being random chance.

ws3 read this as implying the rotating disk-like distribution is not improbable (more than a 1 in 100 chance). However, this 1.4 percent probability is not the probability or a random distribution appearing to be a rotating disk-like structure, it is simply the probability of dwarf galaxies in a disk-like structure (which itself may be improbable) to have random velocities consistent with the observed net rotation. I was simply trying to make the point that, once the spatial distribution is factored in, the observed rotating disk-like structure is inconsistent with a random distribution at far greater than the 1.4 percent level.

Indeed.

To begin with, the authors noticed that 15 of the 27 dwarf galaxies appeared to lie within the same plane. They show that if these 15 dwarf galaxies were randomly distributed around Andromeda, then the chance of them being visually aligned in this manner is only 0.13%. The authors then investigate the radial velocities (motion towards or away from us) to show that 7 out of 8 of those galaxy residing north of Andromeda are moving away from us while 6 out of 7 in the south are moving towards us. The probability that 13 (or more) out of the 15 objects would by random chance have such a kinematic distribution as to suggest rotation is then 1.4%.

So, the total probability of both being placed in this alignment and then having this radial velocity distribution is 0.0013*0.014 ~ 0.00002 ~ 0.002%. Or in other words, as the paper puts it: "Thetotal significance of the planar structure is approximately 99.998%." That is slightly above 4-sigma.

I'm not a Pro, but after what I learned about the alternative, more accularte Theory (for the Universe, Galaxies etc.) the "Plasmaverse" (Electric Univesere) Theory I think those are just baby galaxy's that where born in the galaxy itself and then moved outwards over time while growing.

In the sum there are four videos. My guess is that if you take what they say in the electric universe theory model the formation of the galaxy's are pretty well explained. My enlish isn't best, hope you could follow me sofar. I'm no scientist, just a guy who is very interested in the Universe and openminded theory's

the authors estimated only a 1.4 percent probability of motion like this being random chance.

ws3 read this as implying the rotating disk-like distribution is not improbable (more than a 1 in 100 chance). However, this 1.4 percent probability is not the probability or a random distribution appearing to be a rotating disk-like structure, it is simply the probability of dwarf galaxies in a disk-like structure (which itself may be improbable) to have random velocities consistent with the observed net rotation. I was simply trying to make the point that, once the spatial distribution is factored in, the observed rotating disk-like structure is inconsistent with a random distribution at far greater than the 1.4 percent level.

Thank you! We should compare taking samples out of disks with taking samples out of disk-like configurations to get the actual likelihood, is what I think you (and implicitly Jarron) say. It didn't occur to me that is needed here.